21. 合并两个有序链表

方法1: 双指针迭代

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                cur.next = l1;//直接接上
                l1 = l1.next;
            } else {
                cur.next = l2;//直接接上
                l2 = l2.next;
            }
            cur = cur.next;
        }
        //当一个链表走空了，那么直接把剩下的一个接上就行，因为都是升序的
        cur.next = l1 == null ? l2 : l1;
        return dummy.next;
    }
}

方法2:递归

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        if (list1.val <= list2.val) {
            list1.next = mergeTwoLists(list1.next, list2);
        } else {
            list2.next = mergeTwoLists(list2.next, list1);
        }
        return list1.val <= list2.val ? list1 : list2;
    }
}